1. Field of the Invention
This invention pertains to drilling of wells in the earth. More particularly, apparatus and method are provided for controlling the direction of a drill bit using a Rotary Steerable System (RSS) having a shape memory alloy (SMA) for applying the controlling force.
2. Description of Related Art
Directional drilling in the earth has become very common in recent years. A variety of apparatus and methods are used. Hydraulic motors driven by a drilling fluid pumped down the drill pipe and connected to a drill bit have been widely used. Directional control is achieved by using a “bent sub” just above or below the motor and other apparatus in a bottom-hole assembly. In this mode of drilling the drill pipe is not rotated while direction is being changed; it slides along the hole. More recently, the use of “Rotary Steerable Systems” (RSSs) has grown. These systems are of two common types: “push-the-bit” and “point-the-bit” systems. The drill pipe rotates while drilling, which can be an advantage is many drilling situations such as, for example, when sticking of drill pipe is a risk.
An RSS using the “point-the-bit” method is disclosed in U.S. Pat. No. 6,837,315. The system includes a power generation section, an electronics and sensor section and a steering section. In the power generating system, a turbine driven by the drilling fluid drives an alternator. The electronics and sensor section includes a variety of directional sensors and other electronic devices used in the tool. In the steering section, the shaft driving the bit is supported within a collar and a variable bit shaft angulating mechanism, having a motor, an offset mandrel and a coupling, is used to change the direction of the bit attached to the shaft. Similar power generation and electronics sections are common to many rotary steerable systems.
An RSS using the “push-the-bit” method is disclosed in U.S. Pat. No. 6,116,354. Thrust pistons are attached to pads and when the thrust pistons are actuated the pad is kicked against the wall of the borehole. Hydraulic fluid driving the pistons is controlled by a battery-driven solenoid.
A simpler and more reliable actuation mechanism is needed for driving the mechanisms of both “point-the-bit” and “push-the-bit” systems. This mechanism should provide the force necessary for a wide range of drilling conditions.